The present invention is directed generally to the direct analysis of solids and particularly to a radio frequency powered glow discharge sputtering source for non-conducting solids analysis as well as metals, alloys, semiconductors and the like.
The application of conventional d.c. powered glow discharge devices for the direct analysis of conductive solids such as metals, alloys and semiconductors is well known in the art. Generally, these glow discharge devices are low pressure, inert atmosphere plasmas which rely on cathodic sputtering to atomize solid samples into a "negative glow" region where subsequent excitation and ionization can occur. Most commonly, the conducting sample being analyzed takes the form of a cathode. In a diode design, the sample cathode and an anode sleeve are housed in a vacuum chamber which usually also acts as an anode and is filled with an inert gas such as argon such that a sufficiently high potential placed across the electrodes causes the gas to break down producing electrons and positively charged ions. The negative field potential attracts the discharge ions which hit the cathode surface ejecting atoms, ions and molecules of the cathode material.
By virtue of the electrical biasing, negatively charged species will be accelerated away from and positively charged species returned to the cathode surface. The vast majority of sputtered particles are not charged and can either diffuse back to the cathode surface or into the negative glow. The percentage of atoms entering the discharge excitation region is a function of the discharge pressure and cathode geometry.
The sputtering process acts as a cascade of elastic collisions with the incoming ion imparting some portion of its kinetic energy, which approaches that of the applied potential, into the cathode material's lattice structure. Provided the sputtering ion has sufficient energy and directionality, the cascade will propagate back to the surface resulting in the ejection of cathode material. Sputter yields, the ratio of the average number of sputtered atoms to incident ions, are a function of the relative masses of the collision partners, the incident angle and energy of the sputtering ion, and the cathode material's binding energy.
Glow discharges are currently employed for elemental analysis by atomic absorption, emission, mass spectrometry and a number of laser-based spectroscopic methods. However, these glow discharge sources have been limited by the requirement that the sample be conductive in nature so that it may act as a cathode in a conventional d.c. diode design. In an effort to analyze nonconducting solids without dissolution, nonconducting powder samples have been mixed with a conducting powder matrix. The resulting powder is pressed into a disc sample, which, because of the conductive portion, allows for the required flow of current, but which also permits the sputtering of atoms of the nonconductive material upon impingement by a discharged ion.
This invention, however, is directed to the analysis of nonconducting materials without matrix modification. The use of a radio frequency discharge in argon to sputter and ionize a solid hollow cathode sample for analysis has been described (Analytical Chemistry, 47 (9), 1528, 1975). However, the hollow cathode geometry requires that the sample itself be machined into a cylinder which, in addition to the considerable labor involved therein, prevents depth profiling analysis.
The present invention solves the problem of placing a nonconducting analyte in solution by omitting this step and analyzing the solid material directly. Additionally, it solves the problem of machining the nonconducting solid into a cylinder for a hollow cathode electrode configuration by allowing for the direct analysis of discs of the nonconducting material. This capability, especially in combination with the direct insertion probe described herein, provides a much simpler, cheaper and easier to operate system than any prior known means for insulator analysis.